Convert an imagePublished on April 15, 2015 / Updated on October 21, 2018

Again, this is a vocabulary specific to color management. Conversion is essential because it serves to keep the same colors (i. e. L*a*b*) from one device to another. An image is converted when its RGB values are modified for slightly different ones to take into account the defects of the destination device. If you are a beginner in color management, the most important thing is to know that Photoshop can take care of everything. But what satisfaction if you take the time to know how to do it yourself: your prints, your images on your site will only be more beautiful !

Since there are more or less large color spaces, ICC profiles for each device, it is now necessary to know how to communicate the right color, the "same color" as much as possible, from one device to another, taking into account their characteristics (their defects). The communication of the "correct color" and the change of the corresponding RGB values is called conversion. Let's see why and how now... knowing that it's a bit like in our exchange office : you have 100 pounds in your possession (as if you were in a given color space, the one of the image) and you want the equivalent in euros (you want to print your file correctly with a given printer and paper) and it's the exchange office and the daily rate (the equivalent of the L*a*b* space therefore our standard) that will give you the equivalent through a conversion.... as if by chance we are also talking about conversion when we talk about money.

How to communicate the "right" color ?

Let's start with a little history. More than twenty years ago, an international organization, the International Color Consortium - ICC - founded by Adobe, Microsoft, Apple, Agfa, Kodak, Silicon Graphics and Sun invented and installed, first on an Apple computer, a fabulous tool: Colorsync. In 1993, ICC profiles - or ICM for Microsoft ® - and the color conversion tools that must necessarily accompany it were invented. However, in 1993, the Colorsync tool was only a conversion tool and not a tool for creating ICC or ICM profiles. So they had yet to be invented !
In fact, all these ICC profile creation and conversion tools have only been effective for the general public since 2000, i.e. since Photoshop is now in version 6 and since companies such as MonacoSystems or X-Rite have created great affordable profile creation software and quality sensors. So it's quite recent.

What is the "right" color ?

We now know that the "right" color is actually an L*a*b* color and therefore the "true" color perceived by a camera for example. However, this "true" color is transcribed into RGB value for obvious practical reasons, but since no camera is perfect, it is unable to see them correctly directly. So each RGB value that must transcribe these L*a*b* colors is tainted with a small error. This error is measured during the calibration of the device in question and held available on a small post-it that now accompanies this photo and is called its ICC profile. To display a photo correctly, we saw on the previous page that we had to assign this ICC profile to this photo. Its RGB values are then correctly interpreted and displayed with the "right" L*a*b* colors.

However, we also saw on the previous page that we could then face an embarrassing technical problem. The RGB values that can be measured with a eyedropper on an image no longer correspond to the displayed colors. We had taken as an example a neutral grey (128, 128, 128) but transcribed in the image in the RGB values 128, 138, 126. Thanks to the assignment of the ICC profile, we saw that the image was displayed correctly because Photoshop, thanks to the ICC profile, knew that for this camera, a value 128, 138, 126 should actually be displayed in 128, 128, 128 so in a neutral grey. But this did not only remove the RGB values by eyedropper, they also contained a dominant value. So at this stage of color management there is a problem of "logic" between what the eyedropper measures and what the eye sees. The eyedropper measures a grey with a dominant tinge and our eye sees a neutral grey. It is therefore necessary to carry out another operation to make everything "logical" again. This operation, as you will have guessed, is called a conversion.

What is a conversion and how is it performed ?

Converting an image is therefore used this time to change the RGB values - and no longer the displayed colors - into other R'V'B' values WITHOUT changing - or as little as possible - the L*a*b* colors of the image so that the colors of a photo are printed as accurately as possible, this time taking into account the printer's defects.

Let's take again our example from the previous page - our IT8 test chart - dedicated to the assignment of a profile to understand. We had opened the image file of this test pattern (figure a. below) in Photoshop. The image displayed in Photoshop on the left is an image displayed without a profile to fully understand the process. Finally without a profile, not quite because Photoshop necessarily needs to assign him a profile (ideally his own). However, in the absence of it, he will force the image to use one at all costs and will therefore attribute to it what is called his RGB working space. If we look "stupidly" at the RGB values of the grey parts in figure a (left below), we read 98, 91, 87. However, in the Adobe RGB color space, this corresponds well to a dark reddish gray since the R value is predominant. So at this point, our scanner file has been assigned a force ICC profile but as it is not the right one, the image is displayed strangely. It's time to give him his! And when you assign it the right ICC profile - my-scanner.icc - everything goes back in order at the display level (figure b. on the right below).

The image to which the correct ICC profile has been assigned is simply displayed correctly, here on the right !

But my image, which is now displayed correctly, has a big handicap. The neutral grays displayed do not correspond at all to the RGB values that I can legitimately expect in the information palette if I place my eyedropper on them. I see very neutral grey but the RGB values correspond to reddish grey (98, 91, 87). So I have to do this famous second operation, this time called a conversion, in order to match the displayed colors with "realistic" RGB values. For this I need to convert the RGB values to other R'V' B' values but this time without changing the L*a*b* colors displayed.

In figure c below on the left, identical to figure b above on the right, I see my image correctly displayed but still with the ICC profile of my scanner. By the Image/Mode/convert to profile menu in Photoshop up to PS CS2 or by the Edit/Converted to profile menu in the following Photoshop, I will convert the RGB values of this image into a fairly large color space like the well-known Adobe RGB (figure d.) This time, the L*a*b* color display does not change BUT the associated RGB values change (R'V'B') to become "almost" neutral 109, 110, 110. I'm saying I threw away my image. In the Adobe RGB color space, RGB values 109, 110, 110 translate into a neutral gray since it is a neutral space.

Once converted into a neutral space, I have a visual correspondence between my image and the RGB values.

Now that my image is correctly displayed and in a neutral and large working space, I can touch it up as I wish - possibly with a eyedropper -.

In fact, each device, each image etc.... has its own ICC profile and can be interpreted by software such as Photoshop. It can assign a profile to an image or "translate" the colors, we say convert an image, from one device to another. The devices can therefore communicate via the CMM - color conversion engine - and their ICC profile. In Photoshop since version 6, it is called: ACE color engine. There wasn't any before this version in Photoshop (so it was a long time ago!)

To work, the CMM needs to know which ICC profile is assigned to an image - source ICC profile - to know which colors L*a*b* it has to do for given RGB signals and to which device to send it - destination ICC profile -, therefore convert it into R'V'B' signals.

The CMM is the hub and is based on the L*a*b* colours (the universal standard) and not on RGB or CMYK signals because, as we have seen several times, the L*a*b* colours are absolute. It knows that it must transmit as information such or such color L*a*b* (therefore absolute) and not such or such RGB value (relative) thanks to the ICC profiles and knows which RGB or CMYK signal it corresponds to for this one and only this one. It will therefore translate this L*a*b* color, a different RGB value for each device - even if they are obviously close enough - into another signal R'V' B' or C' M' J' N' so that the destination device reproduces the same L*a*b* color. And the strongest thing is that if he can't do it directly, he will replace it with another one without betraying the perceived visual sensation. This is especially his great strength! This "translation" operation is called a conversion and there are four different ways to do it according to the desired rendering. We will study this in detail on the next page.

When must be a photo converted ?

So we need to convert an image at least once in its history to match what our Photoshop eyedropper measures and what we see. Thereafter, we will automatically make or undergo further conversions to modify the RGB values of our photo - so always without changing the colors or as little as possible - to broadcast this photo on the Internet (sRGB color space), to be displayed correctly on our calibrated screen (this is done on the fly in the graphics card) and therefore take into account its own display defects, or each time it is printed.

Important ! When converting to a neutral color space, you should choose it a little larger than the ICC profile of the device you used. That's why there are many of them, from the smallest sRGB to the largest ProPhoto RGB.

How is the conversion done ?

The paragraph above has just seen the different roles of conversion. When you choose to do a conversion, what really happens to the RGB values of my image ? How can the conversion tool, the CMM, best preserve the L*a*b* colors, i.e. the visual sensation of the photo if the destination device does not allow it, as is unfortunately often the case with printers ? The conversion being in a way a translation whose mission is not to distort the original...

More or less large colour spaces...

As we have seen on the page dedicated to color spaces, the colorimetric spaces called peripheral or working spaces, are more or less large. Some are so large that they include all the others (ProPhoto RGB or some digital cameras). Some are so small that they are encompassed by all the others like some printers on matte paper. But sometimes, however, some are a little larger in one colour or another and vice versa for another. This can be seen in the illustration on the right. Overall, the color space contained in the ICC profile of this printer/paper pair is smaller than the gamut of the screen except towards the blue-green ones. These colors are then printable but invisible on the screen (not displayable as they are, therefore displayed but less saturated than their original saturation). It is more often the opposite, in fact.

In other words, some colors may be contained in the original file but not printable (or even displayable). So we observe two scenarios :

Either the original colors can be printed (in the destination gamut),

Either the colors of the original file are not printable (out-of- gamut).

In the first case, if the colors of the photo are printable, the conversion is only used to change the RGB values of the photo to obtain the same printed color on the print. So far, so good ! But what if the second case or if the gamut of the image is much larger than the gamut of the printer or... the opposite ?

Are they really impossible to display ?

Of course not ! In fact they are "replaced" by the colour or colours - see the next page on conversion modes - the closest and therefore the most saturated one on this screen. On a print run, the colours don't disappear either! They are just printed with the closest and most saturated colours that the printer and its inks can make. So where we eventually had a gradient of colors on the screen (for example), we will have a solid, less saturated, on the print. In fact, with Glossy papers, it's more often the other way around !

What to do with non-printable or out-of-gamut colours ?

Let's assume that the profile of your image is the profile - 1 (red) - and that of your printer - 2 (white) -. Your image has green/yellow areas that the printer will not be able to reproduce because they are called out-of-gamut. No combination of CMYK can reproduce exactly this L*a*b* color belonging to my image (whether I can display it on the screen or not). So how do we do that ? The conversion then consists in "bringing", as with a shoehorn, these greens into the printer's space so that they can still be printed when normally the printer does not know how to do it! However, the visual impression must remain as close as possible to the visual sensations of the original image. To do this shoehorn work, the color management and conversion tools use four conversion rules - only two of which are used for photographers :

The relative mode

The perceptual mode

They are discussed on the next page : relative and perceptual conversion modes. There will inevitably be losses, but a good engine will reduce them as much as possible (losses!) without distorting the visual sensations of the original image. Photoshop's is particularly powerful if you choose the right conversion mode. Of course, some image editing software, which is much cheaper, also has editing tools, such as stamps and other high quality tools, but none have such good color management. Unfortunately, this has a price...

... Et concretely ?

1 - In your camera body (in JPEG)

The assignment of the ICC profile of your box has necessarily taken place automatically in your box. The conversion to a neutral color space like the famous sRGB will also be done directly in your camera if you shoot in Jpeg. To do this, go to the menus of your camera and choose your color space between two options (very rarely three) :

sRGB (by default) Adobe RGB (or Adobe RGB but it's the same).

Attention ! This choice only applies to Jpeg files, even if you are shooting in RAW + Jpeg.

2 - In your demosaicing program (Camera Raw, DXO, etc.) for RAW files

Assigning your box's ICC profile automatically applies when you open your RAW file in your software. You have nothing to do and nothing else you can do at this point ! If your box is not recognized by your software because it is very recent or you have not updated your software database, you will simply not be able to open your RAW file.

Once opened with the right ICC profile, you must choose the neutral color space of destination, as on your box (see above), but with a difference in size : you can choose more options including the very interesting and very useful ProPhoto, in some cases. Example with Photoshop Camera Raw below :

At the very bottom of the window, you can choose the destination color space. In Camera Raw, you can choose between the classic sRGB, the largest Adobe 98 and ColorMatch RGB and the very large ProPhoto.

Important update ! Since the update of Photoshop CC 14.1, it is now possible to choose your destination space from all its profiles! If you have installed them on your computer, you can choose a DON RGB, BEST RGB, MELISSA RGB, etc. and no longer only among four possibilities and in addition, you now have the choice of conversion mode. (Until now, the default choice was relative, which is quite logically explained). We can say that now the situation is perfect and finally leaves us the choice of controlling our color management in Camera Raw.

When you click on the "Open an image" button, your photo will necessarily have this color space.

A true color (L*a*b*) can be "translated" by a very wide variety of RGB values, depending on the device. An RGB colour is therefore not a "true" colour but only a color for a given device and therefore in a given color space.

To communicate the same color (meaning L*a*b*), two devices must exchange RGB values through a conversion : the RGB values of one color are converted into R'V' B' values for another device and therefore for the same color L*a*b*.

This conversion therefore strives to preserve the appearance and colors of a photo as best as possible, even if some colors are not printable, for example.

There are two "ways" to solve these non-printable color problems : the perceptual and relative colorimetric rendering intent discussed on the next page.

This conversion can be done directly once in your camera if you work in Jpeg, in your demo software if you work in RAW and in Photoshop or your image editing software afterwards.

Through these 10 pages we will learn all the vocabulary related to color management: color spaces, ICC profiles, gamuts, etc...

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